Physical and Chemical Changes

Q45. Classify the changes involved in the following processes as physical or chemical changes:

Ans. (a) Photosynthesis - chemical change

(b) Dissolving sugar in water - physical change

(c) Burning of coal - chemical change

(d) Melting of wax - physical change

(e) Beating aluminium to make aluminium foil - physical change

(f ) Digestion of food - chemical change

(g) Condensation of steam – physical change

(h) Spoiling of food – chemical change

(i) Burning of coal – chemical change

Q46. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Ans. When a candle burns, both physical and chemical changes take place.

The melting of the solid wax to form liquid wax and the evaporation of liquid wax to form wax vapour are physical changes. The burning of the wax vapour is a chemical change.

Example

Burning of LPG involves both physical and chemical changes because when LPG comes out of cylinder and is converted from liquid to gaseous state it is a physical change. When the gas burns in air it is a chemical change.

Q47. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Ans. Burning of wood

Burning is a non-reversible chemical change because when we burn wood new substances are formed as the carbon in the wood reacts with oxygen in the air to create ash and smoke, and energy in the form of light and heat.

Cutting of wood it into small pieces

Cutting of wood into small pieces are physical change as no new substance is formed. Only shape and size changes when wood is cut into small pieces.

Q48. What happens when an iron nail is dipped in copper sulphate solution?

Ans. When an iron nail is dipped in the copper sulphate solution (CuSO₄) then iron displaces copper from the copper sulphate because iron is more reactive than copper. The change of colour of the solution from blue to green is due to the formation of iron sulphate (FeSO₄), a new substance. The brown deposit on the iron nail is copper, another new substance. We can write the reaction as:

Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)

Q49. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

Ans. (ii) Process – B is a chemical change.

Q50. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

Ans. (iii) Both processes A and B are chemical changes.

Q51. Differentiate between physical and chemical change.

Ans. Difference between physical and chemical change

Q52. Describe how crystals of copper sulphate are prepared.

Ans. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Continue adding copper sulphate powder till no more powder can be dissolved. Filter the solution. Allow it to cool. Do not disturb the solution when it is cooling. Look at the solution after some time. You see the crystals of copper sulphate.

Image from NCERT